This invention relates to methods and apparatus for transferring wafers of the kind used in the manufacture of semiconductors between cassettes and a wafer carrier boat of the kind used in a furnace.
This invention relates also to methods and apparatus for loading and unloading a boat into and out of a vertical furnace.
Wafers are made of a substrate material (e.g. silicon) which is relatively brittle. The boat used for supporting the wafers within a heating and/or processing chamber of a furnace is also usually made of a material (e.g. quartz) which is relatively brittle. And the coatings on the wafers and the boat are often brittle enough to be rather easily fractured. All of these materials are therefore subject to chipping or flaking if an edge of a wafer hits a cassette or boat in the course of transferring the wafer between a cassette and a boat.
The transfer of wafers between a boat and a cassette must be done with great precision in order to avoid any chipping or flaking contact between an edge of the wafer and a cassette or a boat.
If any such contact is made, and if any material is chipped or flaked off of a wafer or a boat, the electrical circuits on the wafer can be damaged. The circuits are densely packed together and have conductivity lines or paths which are of such extremely small size that any foreign material, such as particles of silicon, quartz or coatings, cannot be tolerated.
The wafers which are to be loaded in the boat and heated in the furnace are supplied to the furnace loading system stacked in slots in cassettes. The wafers must be transferred from these cassettes to the boat prior to the placing of the boat in the heating chamber of the furnace, and the wafers must be removed from the boat and returned to cassettes after the wafers have been subjected to a heating and/or processing cycle in the furnace.
In transferring a wafer between a cassette and a boat it is desirable that the transfer be done by an automated mechanism which can operate in a clean environment without manual intervention.
It is also generally desirable that a single boat be large enough to handle the contents of a number of cassettes, because a substantial amount of time is usually required for each process cycle. Processing more wafers at one time increases the production rate.
In order to be able to load a relatively large number of wafers from a number of different cassettes into a single boat, the loading mechanism has to be able to move a wafer vertically, radially and in a circular arc in the course of picking up a wafer from a source (a cassette or the boat) and transferring the wafer to a destination (a boat or a cassette).
The mechanism for transferring the wafer must therefore be precise in its construction and operation.
It must also be able to accommodate the variations or differences in the actual positions of wafer centers from the expected or the calibrated positions of wafer centers resulting from dimensional tolerances that are inherent in the system. This variation or offset of wafers centers must be determined and effectively corrected if the unacceptable edge contact between the wafers and the boat and cassettes is to be eliminated during the transfer operations.